Your invitation must have been lost in the mail, but last month Tom Cruise celebrated his 42nd birthday. Tom Hanks blew out candles this summer as well – he turned 48. These days, pretty much all of Hollywood’s most famous heavyweights (Meg Ryan, 42; Mel Gibson, 48; Bruce Willis, 49; Denzel Washington, 49; John Travolta, 50; Harrison Ford, 62) are ambling around their middle ages – or rapidly hurtling toward them (Julia Roberts is going on 37).

Not that there’s anything wrong with that. But it’s bracing all the same, watching this entire generation of actors – the generation that climbed to the top of the A list way back in the 1980s and has dominated it ever since – shuffling inexorably toward AARP eligibility. And it raises an obvious, unavoidable question: Who is replacing them? Who among Hollywood’s next gen of actors – the litter now in their 20s and early 30s, with names like Ben and Colin and Reese – has proven themselves huge enough to fill their enormous Uggs? Orlando? Keira? Cameron?

So far, the answer looks like a big fat None Of The Above.

The fact is, the most powerful box office stars continue to be the same ones who dominated in 1994 (actors with names like Tom and…Tom). With two possible exceptions, Hollywood hasn’t made a major new discovery in a decade (those would be Russell Crowe, who turned 40 last April, and Will Smith, age 35). There have been lots of extraordinary performances, to be sure (from Nicole, Renee, and Jude, to name a few), but that just means there are plenty of great young actors at the moment, not true stars, at least not by any traditional definition of the term.

None of these kids, for instance, have shown that they can open a movie the way Hanks usually can (except maybe Adam Sandler, but then comedians only open comedies – see Mike Myers and Jim Carrey). None have demonstrated the range to zigzag from drama to action to romance the way Cruise continues to. Certainly none have established that they can galvanize a global audience across a multitude of cultures and demographics – the way Julia Roberts has been doing since she was in her 20s – and deliver them to theaters for film after film (at least not without playing a hobbit, a wizard, or an arachnid-enhanced superhero). Frankly, Hollywood just isn’t supersizing movie stars the way it used to.

”The thing that’s up for debate is, Is it just a cycle or has something changed about the way movies are made?” asks screenwriter Dean Georgaris (”The Manchurian Candidate,” ”Mission: Impossible 3”). ”It’s something that writers and producers talk about. Studio execs talk about it. Directors talk about it. Where are the movie stars? And we talk about it because it is still, right now, considered a star-driven business.”

One of those studio execs (the head of a major one, actually) has an off-the-record theory. ”The younger generation hasn’t found its movie stars yet, except in the tiniest ways. And you can put a lot of the blame on us. We make such meaningless movies nowadays. Everything is so packaged and disposable. Why would anybody make an emotional investment in these films? Why would anybody fall in love with the people in these movies? And you need that to become a star – you need people to feel something when they see you on the screen.”